Belatedly occurred to me that not everyone here is an engineer like Leigh, who knows how to read technical graphs. So here, quick and dirty, is what you should be getting out of the graphs in the article in my book.
The first graph shows the light spectrum coming from the enlarger. The top curve is data provided to me by Beseler for the particular enlarger I owned. In this particular case it's actually pretty close to a blackbody curve, but that can be very different for different enlargers. That's the light spectrum that gets to the lens. The curve below it shows what happens to the spectrum after it goes through the enlarging lens (using manufacturer-supplied transmission data for that lens). As you can see, that filters out most of the really short stuff, but there's still some stuff getting through down to 360 nm.
The bottom three curves show the effect on that light of adding in the sharp UV-cutting filters. They remove progressively more of the extremely short wavelengths but don't effect the longer ones much. Those are the filters I use for producing the results in the two tables in the article. Observe that in the first table the exposure time goes up markedly as progressively stronger filters are used, far more than it would if there were no very short wavelength response in the paper. That confirms that there's a lot of paper response below 420 nm (violet). Observe that in the second table the amount of focus shift goes down as the amount of short wavelength light is reduced.
This pretty much nails it that the print paper is responsive to extremely short wavelengths (which ARE present in the light) and that this produces a focus shift. The question remains why don't we see this when we're focusing by eye and so correct for it?
For that, you look to the second graph, which shows that in this situation the peak short wavelength response to the print paper is very different from the visual response. There's a good 50 nm separating them; in fact the peak visual response when focusing with blue light falls near a minimum in sensitivity for this paper.
Combine that with the fact that enlarging lenses are generally poorly corrected for longitudinal chromatic aberration (focusing different colors at different distances) and that this is even more true when one gets to extremely short wavelengths, it's easy to understand the mismatch between what looks like our sharpest focus point what the paper finds as the sharpest focus point.
The final two graphs expand upon this information but you don't have to understand them to understand the problem.
Again, this is a short summary. Please review the original article in POST EXPOSURE before making assumptions about what I did or didn't do.
pax \ Ctein
[ Please excuse any word-salad. MacSpeech in training! ]
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Last edited by Ctein; 11-30-2011 at 10:52 PM. Click to view previous post history.